A nacelle is the housing for a turbofan gas turbine engine used to power, for example, a commercial airliner. The nacelle and the engine together are referred to as the propulsion system or power plant of the aircraft. The nacelle forms the external aerodynamic surfaces of the propulsion system, and also helps form the duct for the bypass air from the fan, as well as encloses all the components and auxiliary devices surrounding and attached to the engine. The nacelle may also optionally provide a reverse thrust means to generate reverse thrust to slow the aircraft, for example during landing. FIG. 1 illustrates a typical gas turbine engine inside a nacelle 22, which are together attached via a pylon 21 to the underside of an aircraft wing 20. The nacelle 22 includes a forward section 23 and an aft section 12. The forward section 23 may be formed from an inlet and a fan cowl, which could be combined together. The aft section 12 may include a thrust reverser.
In operation, an ambient air flow 56 on the exterior of the nacelle 22 generates drag. The drag force increases fuel consumption. Accordingly, it is desirable to minimize the drag.
If the flow around the nacelle 22 is laminar the drag force will be reduced compared to a turbulent flow. Aircraft and nacelle designers have utilized nacelle external aerodynamic shapes that maintain a natural laminar flow over a portion of the first section 23 of a nacelle. By carefully selecting the aerodynamic profile (which is done through analysis including computational fluid dynamics, and through experimentation), natural laminar flow can be achieved beginning at the inlet lip and extending back as much as 10-30 inches. The portion where it is desired to achieve laminar flow much be clean of debris (e.g., bugs) and very aerodynamically smooth (e.g., fastener heads must be very flush and parallel with the surrounding surface). Steps and gaps can cause the laminar flow to trip and transition to turbulent flow. Many other active or hybrid natural/active techniques have been proposed for achieving and maintaining laminar flow on an aircraft nacelle and other aircraft surfaces. One such technique is through boundary layer ingestion or suction where the boundary layer next to the aircraft surface is pulled through small holes in the surface to remove the low energy boundary layer and regenerate it or maintain it at a minimum energy level. Active or hybrid laminar flow techniques may achieve and maintain laminar flow more consistently than natural means alone, and may be able to extend further aft the region on first section 23 of nacelle 22 which has laminar flow.
While boundary layer suction or ingestion has been proposed for use on an aircraft surface such as an aircraft nacelle for drag reduction, no successful commercial systems have flown yet. There is a need for solutions to the many practical problems that remain to be solved.